The present invention relates to a switching power supply apparatus. More particularly, it relates to a switching power supply apparatus in which a rectifying current that flows in a primary side of a transformer connected to a load side is formed to provide nonlinear characteristics when a power supply is started up, thereby preventing an excessive current from flowing in a switching transistor or the like provided at the primary side of the transformer.
As a switching power supply apparatus, there is known an apparatus based on a current resonance system. FIG. 1 shows a conventional example of switching power apparatus of this current resonance system, the apparatus having a SEPP (Single Ended Push Pull) arrangement.
A switching power supply apparatus 10 shown in FIG. 1 comprises switching signal generating means 12 including a variable frequency oscillating circuit 14 and a drive circuit 16. The oscillating circuit""s oscillation signal is supplied to the drive circuit, thereby generating a pair of switching signals, for example, having a reverse phase relationship therebetween. In the case where the switching generating means 12 is composed of IC circuits, an oscillating element (capacitor 18 and resistor 20) that determines an oscillation frequency is externally provided at any of external terminals 12a and 12b of this IC circuit.
A pair of switching signals Sp, Sp bar are supplied to a pair of switching elements 22 and 24 having SEPP arrangement. A MOS type electric field effect transistor or the like may be utilized as the switching elements 22 and 24. An resonating capacitor 28 is connected to both a ground and a connection neutral point xe2x80x98pxe2x80x99 between a pair of these switching elements 22 and 24 via a primary coil 26a of an insulation transformer 26.
Respective diodes 30a and 30b rectify a secondary current that flows in a pair of secondary coils 26b and 26c of the insulation transformer 26 as full-wave rectifier. The full-wave rectified current allows a smoothing capacitor 32 to be charged. Therefore, voltage xe2x80x98vbxe2x80x99 obtained at both ends 34 of the smoothing capacitor 32 is supplied to a load (not shown) as an output voltage.
The output voltage is supplied to an amplifier 36, as voltage comparison means, wherein the voltage is compared with a reference voltage, Vref Its comparison output is supplied to a photo-coupler 38 that configures inductance control means 37 provided in order to insulate the primary and secondary sides of the transformer 26. The photo-coupler 38 comprises a photodiode 40 and a phototransistor 42 that functions as a variable inductance element. A current based on the comparison output flows in this phototransistor 42.
The phototransistor 42 is connected to the external terminal 12b through a stationary resistor 44. Therefore, when the phototransistor 42 is ON, this resistor 44 and serial impedance caused by the phototransistor 42 are connected in parallel to a resistor 20, which is an oscillation element.
In this arrangement, it is known that a relationship between a resonation frequency xe2x80x98fxe2x80x99 and a resonation impedance Z of the resonating circuit on the primary side of the transformer 26 formed of its primary coil 26a and the capacitor 28 is based on upper side operation as indicated by a curve xe2x80x98Loxe2x80x99 in FIG. 2.
In this resonating circuit, when switching frequencies of the switching signals Sp and Sp bar supplied to a pair of switching elements 22 and 24 are increased, the resonance impedance Z increases. The resonance impedance Z is lowered as the switching impedance Z is lowered. Such change in the resonance impedance Z causes a resonance current i1 that flows in the primary coil 26a to be changed. Thus, controlling this resonance current i1 allows an output voltage Vb induced at the secondary side of the transformer 26 to be controlled.
When the output voltage Vb obtained at an output terminal 34 is illustratively higher than the reference voltage Vref, the phototransistor 42 has its impedance according to the comparison output. Thus, the composite resistance of the external terminal 12b becomes smaller than a case of a simplex of the resistor 20, whereby an oscillation frequency xe2x80x98fswxe2x80x99 increases.
When the oscillation frequency xe2x80x98fswxe2x80x99 is increased, the resonance impedance Z determined depending on the primary coil 26a and the capacitor 28 increases. Thus, a current that flows in this primary coil 26a is limited, and its value decreases. With this decrease in current, the currents induced at the secondary coils 26b and 26c are reduced as well. As a result, a charge voltage with the capacitor 32 decreases. Namely, the output voltage Vb is controlled in the direction of the reference voltage Vref.
Conversely, when the output voltage Vb is lower than the reference voltage Vref, the impedance of the phototransistor 42 increases, and the composite resistance value at the external terminal 12b increases. Then, the variable frequency oscillating circuit 14 is controlled so that its oscillation frequency xe2x80x98fswxe2x80x99 may be lowered. As a result, the switching frequency is lowered relevant to the switching elements 22 and 24, and the primary resonance impedance Z of the transformer 26 is lowered accordingly. This causes the resonance current to increase. When the resonance current increases, the secondary current increases as well. Thus, the charge voltage Vb with the capacitor 32 rises, and a closed loop control is performed so as to be close to the reference voltage Vref.
In the meantime, in this switching power supply apparatus 10, a large amount of resonance current flows from a time when a power supply is turned ON to a time when the capacitor 32 rises to a voltage in its constant state. Thus, this current may damage the switching elements 22 and 24.
In order to reduce such damage, there has been conventionally provided a soft start circuit 50, which functions as frequency control means 60, for limiting a resonance current during startup. This soft start circuit 50 is provided in the switching signal generating means 12. An external charging capacitor 52 is connected to an external terminal 12c arranged at this soft start circuit 50, so that charging for this capacitor 52 is started in synchronism with turning ON the power. Then, a change in charge voltage Va at this time causes a charge current of the oscillating capacitor 18, which is an oscillating element, connected to the external terminal 12a to be changed.
When a charge current with the oscillating capacitor 18 changes with an elapse of time, the oscillation frequency xe2x80x98fswxe2x80x99 changes accordingly. This fact will be described with reference to FIGS. 3A to 3E.
FIG. 3A shows a change in charge voltage Va when and after the power is turned ON, wherein the charge characteristics are linear as indicated by line La. The variable frequency oscillating circuit 14 is changed in the oscillation frequency xe2x80x98fswxe2x80x99 by the charge voltage Va of the capacitor 52 associated with the soft start circuit 50 connected to the oscillating circuit 14. The oscillation frequency fsw changes almost linearly as indicated by characteristic line Lb in FIG. 3B. When a charge voltage Va is zero volt, oscillation occurs at a high frequency, and the oscillation frequency xe2x80x98fswxe2x80x99 is lowered as the charge voltage Va increases.
On the other hand, the primary resonance impedance Z is characterized by characteristic curve Lo such that the resonance impedance Z increases as a frequency increases from the resonance frequency xe2x80x98foxe2x80x99 as shown in FIG. 2. A relationship between the impedance Z and a time is illustrated as shown in FIG. 3C. Namely, there are nonlinear characteristics that the resonance impedance Z is initially high, and then, lowers rapidly; the impedance gently changes as the charge voltage Va is close to a full charge.
As a result, there is provided nonlinear characteristics such that, although not so much primary current i1 flows in this primary resonance circuit system from a time when the power is turned ON to a predetermined time, as indicated by curve Lc in FIG. 3D, the current il increases rapidly after a certain period of time has elapsed. Accordingly, there is established a charge mode in which, although the output voltage (charge voltage) Vb of the capacitor 32 connected to the output terminals 34 is initially charged gently as indicated by the curve Ld in FIG. 3E, rapid charging is then performed. Immediately before a time xe2x80x98tbxe2x80x99 when a soft start mode terminates, the result is gentle charging; and at the time and after the time xe2x80x98tbxe2x80x99 this mode transits to a voltage control mode caused by a closed loop. In this control mode, voltage control is performed such that the reference voltage Vref is obtained as indicated by the line Le in FIG. 3E.
Thus, a rapid current ii flows in the primary resonance system until a time has come immediately before the soft start mode terminates because of an effect due to a change in the primary impedance Z. This rapid current il causes a pair of switching elements 22 and 24 an excessive stress, and thus, the switching elements 22 and 24 or the like
Although a voltage change applied to a load while the power is ON depends on charge characteristics of the soft start circuit, a voltage applying state that is the most suitable to the load can be achieved if the voltage change state matching such load can be freely set. However, conventional art as described above has been such a disadvantage that flexible response cannot be made, since the charge characteristics of the soft start circuit is merely linear.
Accordingly, it is an object of the present invention to provide a switching power supply apparatus in which the charge characteristics of a capacitor connected to the soft start circuit are made gentle when the power is turned ON, whereby damage to at least the switching elements can be reduced.
In accordance with the invention, the object is accomplished in switching power supply apparatus comprising a frequency control device, preferably such as a soft start circuit and a charging capacitor, for controlling an oscillation frequency when switching signal generating device, preferably such as variable frequency oscillating circuit is initially driven. A frequency control signal of the frequency control devices is formed to provide nonlinear characteristics relevant to a time. In carrying out the present invention in one preferred mode, since the charge characteristics of the soft start circuit are made nonlinear, charging for a capacitor connected to the soft start circuit is rapidly performed when the power is ON, and then, the charging is performed gradually.
By doing this, the primary impedance Z that originally changes nonlinearly changes almost linearly. This resonance impedance Z determines the primary current i1, and thus, the excessive current of this primary current i1 is inhibited. Therefore, no excessive current flows in switching elements, and damage to these switching elements can be reduced.
In addition, an output voltage to be applied to a load, particularly a voltage change when power is ON depends on the charge characteristics of a soft start circuit. The charge characteristics are provided as charge characteristics suitable to the load, whereby more stable circuit operation can be achieved.
According to the present invention, the switching power supply apparatus involves a pair of switching elements for receiving the switching signal, a resonating capacitor connected to a connection point of a pair of these switching elements via a primary coil of a transformer, a rectifying circuit provided at a secondary side of the transformer, comparison devices, preferably such as an amplifier, for comparing an output voltage obtained at this rectifying circuit with a reference voltage, and impedance control devices, preferably such as a photo-coupler, for controlling an impedance of an oscillation element of the variable frequency oscillating circuit based on this comparison output.
The switching power supply apparatus according to the present invention is very preferable on applying it to a switching converter having a SEPP configuration or the like.
The concluding portion of this specification particularly points out and distinctly claims the subject matter of the present invention. However those skilled in the art will best understand both the organization and method of operation of the invention, together with further advantages and objects thereof, by reading the remaining portions of the specification in view of the accompanying drawing(s) wherein like reference characters refer to like elements.